The invention relates to the field of sports involving gliding over snow, for example alpine skiing, cross-country skiing, snowboarding and monoskiing or the like. It more precisely concerns an improvement to the internal structure of the boards for engaging in these sports, improving their mechanical structure and their dynamic behavior.
In the rest of the description, the invention is described more precisely in its application to a piste ski, adaptation to the other types of gliding boards being obvious to the person skilled in the art.
As is known, a modern alpine ski is made by assembling various superimposed elements.
Thus, in a traditional ski, there is a lower assembly constituting the gliding surface, and an upper assembly including the upper face proper and the side flanks. A core, made either of wood, a honeycombed structure or foam, in particular polyurethane foam, is interposed between these upper and lower assemblies.
In "monocoque" skis, the upper assembly consists of the upper layer, the extensions of which are inclined to form the side faces of the ski.
As is known, it proves necessary to incorporate reinforcing elements between the core and the upper assembly, or the core and the lower assembly. These reinforcing elements, generally consisting of textile webs such as glass fiber fabrics impregnated with a thermoset resin, reinforce the mechanical qualities of the ski and thus improve its dynamic behavior.
Of the various methods for making gliding boards, one of the fastest ones is that referred to as "in situ injection". It consists in placing the constituent elements of the shell of the board in a mold, then injecting into them liquid components which react exothermically to form a polyurethane foam whose expansion pressure presses the shell elements against the walls of the mold, said foam also adhering to the constituent elements of the board.
Further, in order to manufacture skis of this type, the reinforcing elements are placed in the molds in a moist state, before the resin intended to give them the mechanical properties has cross-linked. This cross-linking will take place when the polyurethane foam constituting the core expands, by virtue of the heat released in the exothermic expansion reaction.
However, the resin of the reinforcing element and the polyurethane foam components have been found to be chemically reactive, that is to say that bringing them into contact, and therefore mixing them, impairs the cross-linking of the resin and modifies the reaction which leads to the formation of the polyurethane foam.
The problem which the invention proposes to solve therefore consists in avoiding or limiting the onset of undesired chemical reactions between the various chemical elements contained in the adjacent layers constituting the ski.
In document FR 2 654 645, it is proposed to interpose a leaktight plastic sheath between the lower and upper assemblies, into which sheath the polyurethane foam constituents which are intended to expand are injected. In this way, this sheath isolates the reinforcing element impregnated with epoxy resin from the polyurethane foam. Unfortunately, a solution of this type has a number of drawbacks, in particular the stretching of this plastic sheath which, in practice, leads to random deformations of this sheath and therefore inaccuracies in the positioning of the internal elements of the structure. This detrimentally affects the contact and adhesion between the various elements.
However, since a gliding board consisting of an assembly of layers undergoes flexing, it is essential for the layers to adhere perfectly so that they withstand the shear stresses generated by these deformations at the interfaces.
The second problem which the invention therefore proposes to solve is to permit optimum adhesion of the reinforcing element to the polyurethane foam core.
Lastly, it is known that, in behavior on snow, a gliding board made from a stack of rigidly assembled elements gives the sensation of a product which is dry, resilient, hard, harsh and therefore uncomfortable and very tiring.